Due to rapid advance and development of the display technology, a TFT-LCD (thin film transistor liquid crystal display) is preferred when compared to CRT type due to its light weight, thinness depth, small volume, and lower radiation. Presently, the TFT-LCD is the main item in the display market. The main object of the manufacturers is to upgrade its display quality, reliability, and reduce or lower the manufacturing cost.
An amorphous silicon thin film transistor is generally used as a switch for the pixel of an LCD or an organic electric-luminescence device (OELD) since the thin film transistor is capable of controlling the current conductivity.
In recent years, the manufacturers have noted a thin film transistor having dual-gate structure, which is introduced in order to promote the conventional conductivity of the thin film transistors within the LCD. FIG. 1 shows a cross sectional view of a conventional dual-gate transistor fabricated on a substrate 10. The conventional dual-gate transistor includes a first gate 11, a second gate 16, a semiconductor layer 13, a drain 14, and a source 15.
The first gate 11 is formed on the substrate 10. The semiconductor layer 13 is formed on the first gate 11, and includes a channel layer 131 and a doped semiconductor layer 132. The drain 14 and source 15 are substantially corresponding to the two sides of the first gate 11 and are formed on the doped semiconductor layer 132. The drain 14, the source 15, and the semiconductor layer 13 have contacted areas is formed by the doped semiconductor layer 132. The second gate 16, is corresponding to the first gate 11, formed on the drain 14 and source 15, and has left and right end sections covering two ends of the drain 14 and source 15. The second gate 16 is electrically coupled to the first gate 11.
The conventional dual-gate transistor further includes a first dielectric layer 12 formed on the substrate 10 and covering the first gate 11, and a second dielectric layer 17 covering on the drain 14 and source 15 and located below the second gate 16. In other words, the second dielectric layer 17 is sandwiched between the second gate 16 and the drain 14 and source 15.
The above-mentioned conventional dual-gate transistor can provide a better current conductivity when compared to a conventional single-gate transistor, and thus reduces the crowding effect of electric field, thereby effectively improving the photo leakage current problem.
In the conventional dual-gate transistor, the left and right end sections of the second gate 16 covering respectively on the ends of the drain and source 14, 15 lead to increase of the parasitic capacitance (Cgs) at the covering position of the second gate 16 and the source 15 when the LCD is under the operation. Note that the parasitic capacitance increasing results in higher feed through voltage and then affects the operating voltage of the pixel electrode and the accuracy of gray values for displaying an image on the display panel.
It is thus desirable and advantageous to provide a dual-gate transistor that is capable of effectively reducing the occurrence of the parasitic capacitance so as to eliminate the above-mentioned influence.